Onset
of mass transfer some 500 million years ago when the donor object
(right), began losing mass to the compact yet more massive white dwarf
companion (left). At the time of this illustration, the star system
appeared much brighter in optical light than it does today.

The Near Infrared Imager (NIRI) on Gemini North was used to make the
observations used in this research on EF Eridani. The observations
were made from 7:21 until 9:00 UT on December 24, 2002 in queue
scheduling mode. The f/6 camera was used with a four pixel slit and
the K-grism to produce K-band spectra with a resolution of R ~ 780.
Individual spectra with exposure times of 120 seconds were obtained at
five different positions along the slit with additional 6” offsets.
In total, 40 individual observations were obtained in 100 minutes covering 123% of
an orbital cycle of the EF Eri system. Additional spectra were
obtained to remove atmospheric telluric features from the data.

Full paper to be published in the Astrophysical Journal, October 20, 2004. See preprint of paper here.

Astronomers using the
Gemini North and Keck II telescopes have peered
inside a violent binary star system to find that one of the interacting
stars has lost so much mass to its partner that it has regressed to a
strange, inert body resembling no known star type.

Unable to sustain nuclear fusion at its core and doomed to orbit
with its much more energetic white dwarf partner for millions of years,
the dead star is essentially a new, indeterminate type of stellar
object.

"Like the classic line about the aggrieved partner in a romantic
relationship, the smaller donor star gave, and gave, and gave some more
until it had nothing left to give," says Steve B. Howell, an astronomer
with Wisconsin-Indiana-Yale-NOAO (WIYN) telescope and the National
Optical Astronomy Observatory, Tucson, AZ. "Now the donor star has
reached a dead end - it is far too massive to be considered a
super-planet, its composition does not match known brown dwarfs, and it
is far too low in mass to be a star. There's no true category for an
object in such limbo."

The binary system,
known as EF Eridanus (abbreviated EF Eri), is located 300 light-years
from Earth in the constellation Eridanus. EF Eri consists of a faint
white dwarf star with about 60 percent of the mass of the Sun and the
donor object of unknown type, which has an estimated bulk of only
1/20th of a solar mass.

Howell and Thomas E. Harrison of New Mexico State University made
high-precision infrared measurements of the binary star system using
the spectrographic capabilities of the Near Infrared Imager (NIRI) on
the Gemini North telescope and NIRSPEC on Keck II both on Mauna Kea in
December 2002 and September 2003, respectively. Supporting
observations were made with the 2.1-meter telescope at Kitt Peak
National Observatory near Tucson in September 2002.

EF Eri is a type of binary star system known as magnetic cataclysmic
variables. This class of systems may produce many more of these 'dead'
objects than scientists have realized, says Harrison, co-author of a
paper on the discovery to be published in the October 20 issue of the
Astrophysical Journal. "These types of systems are not generally
accounted for within the usual census figures of star systems in a
typical galaxy," Harrison says. "They certainly should be considered
more carefully."

The white dwarf in EF Eri is a compressed, burnt-out
remnant of a solar-type star that is now about the same diameter as the
Earth, though it still emits copious amounts of visible light. Howell
and Harrison observed EF Eri in the infrared because infrared light
from the pair is naturally dominated by heat and longer wavelength
emissions from the secondary object.

The scientific detective
work to deduce the components of this binary system was complicated
greatly by the cyclotron radiation emitted as free electrons spiral
down the powerful magnetic field lines of the white dwarf. The white
dwarf's magnetic field is about 14 million times as powerful as the
Sun's. The resulting cyclotron radiation is emitted primarily in the
infrared part of the spectrum.

"In our initial spectroscopy of
EF Eri, we noted that some parts of the infrared continuum light became
about 2-3 times brighter for a time period, then went away. This
brightening repeated every orbit, and thus had to have an origin within
the binary," Howell explains. "We first thought the brightness change
resulted from the difference between a heated side and a cooler side of
the donor object, but further observations with Gemini and Keck instead
pointed to cyclotron radiation. We 'see' this additional infrared
component at the phases which occur when the radiation is beamed in our
direction, and we do not see it when the beaming points in other
directions."

The 81-minute orbital period of the two objects
was probably four or five hours when the mass transfer process began
about five billion years ago. Originally, the secondary object may also
have been similar in size to the Sun, with perhaps 50-100 percent of a
solar mass.

"When this interactive process of mass transfer
from the secondary star to the white dwarf begin, and why it stopped,
both remain unknown to us," Howell says. During this process, repeated
outbursts and novae explosions were very likely. The physics of the
process also caused the two objects to spiral closer to each other.
Today, the two objects orbit each other at about the same separation as
the distance from the Earth to the Moon. The donor object has regressed
to a body with a diameter roughly equal to the planet Jupiter.

The
combined observing power of the Gemini 8-meter and Keck 10-meter
telescopes and their large primary mirrors, which were essential to
this research, Howell says, makes it clear that neither spectral
features of the donor nor its composition match any known type of brown
dwarf or planet.

Derek Homeier University of Georgia created a
series of computer models that attempt to replicate the conditions at
EF Eri, but even the best of these do not match perfectly.

The
shape of the spectra indicate a very cool object (about 1,700 degrees
Kelvin, equivalent to a cool brown dwarf), yet they do not have the
same detailed shape or key features of brown dwarf spectra. The coolest
normal stars (very low mass M-type stars) are about 2,500 degrees K,
and Jupiter is 124 degrees K. The close-in "hot Jupiter" exoplanets
detected indirectly by other astronomers using their gravitational
effect on their parent stars are estimated to be 1,000-1,600 degrees K.

There
is a small chance that the EF Eri system could have originally
consisted of the progenitor of the present-day white dwarf star and
some sort of "super-planet" that survived the evolution of the white
dwarf to result in the system observed now, but this is considered
unlikely.

"There are about 15 other known binary systems out
there that may be similar to EF Eri, but none has been studied enough
to tell," Howell says. "We are working on some of them right now, and
trying to improve our models to better match the infrared spectra."

Co-authors
of this paper on EF Eri are Paula Szkody of the University of
Washington in Seattle, and Joni Johnson and Heather Osborne of New
Mexico State.

The WIYN 3.5-meter telescope is located at Kitt
Peak National Observatory, 55 miles southwest of Tucson, AZ. Kitt Peak
National Observatory is part of the National Optical Astronomy
Observatory, which is operated by the Association of Universities for
Research in Astronomy (AURA), Inc., under a cooperative agreement with
the National Science Foundation (NSF).

The national research
agencies that form the Gemini Observatory partnership include: the US
National Science Foundation (NSF), the UK Particle Physics and
Astronomy Research Council (PPARC), the Canadian National Research
Council (NRC), the Chilean Comisión Nacional de Investigación
Cientifica y Tecnológica (CONICYT), the Australian Research Council
(ARC), the Argentinean Consejo Nacional de Investigaciones Científicas
y Técnicas (CONICET) and the Brazilian Conselho Nacional de
Desenvolvimento Científico e Tecnológico (CNPq). The Observatory is
managed by AURA under a cooperative agreement with the NSF.

The
W.M. Keck Observatory is operated by the California Association for
Research in Astronomy (CARA), a scientific partnership of the
California Institute of Technology, the University of California, and
the National Aeronautics and Space Administration.

Onset of mass transfer some 500 million years ago when the donor object
(right), began losing mass to the compact yet more massive white dwarf
companion (left). At the time of this illustration, the star system
appeared much brighter in optical light than it does today.

The
present day situation around donor object (right) where the small,
dense white dwarf (center) has "consumed" much of its companion star's
mass and it is now a cool, dark ember about the size of Jupiter. Today
most of the radiation from the system is emitted in the infrared part
of the electromagnetic spectrum.

The
process by which cyclotron radiation is produced as free electrons
spiral around magnetic field lines generated by the white dwarf
companion in the EF Eridanus system. Radiation from this process is
emitted in the infrared part of the spectrum.

Location
of EF Eridanus in the southern constellation of Eridanus, the River.
Although only visible in large infrared telescopes today, some 500
million years ago, this system might have been visible as a dim point
of light to the naked eye.

Onset
of mass transfer some 500 million years ago when the donor object
(right), began losing mass to the compact yet more massive white dwarf
companion (left). At the time of this illustration, the star system
appeared much brighter in optical light than it does today.

The Gemini Observatory is an international collaboration with two identical 8-meter telescopes. The Frederick C. Gillett Gemini Telescope is located on Maunakea, Hawai'i (Gemini North) and the other telescope on Cerro Pachón in central Chile (Gemini South); together the twin telescopes provide full coverage over both hemispheres of the sky. The telescopes incorporate technologies that allow large, relatively thin mirrors, under active control, to collect and focus both visible and infrared radiation from space.

The Gemini Observatory provides the astronomical communities in five partner countries with state-of-the-art astronomical facilities that allocate observing time in proportion to each country's contribution. In addition to financial support, each country also contributes significant scientific and technical resources. The national research agencies that form the Gemini partnership include: the US National Science Foundation (NSF), the Canadian National Research Council (NRC), the Argentinean Ministerio de Ciencia, Tecnología e Innovación Productiva, the Brazilian Ministério da Ciência, Tecnologia e Inovação and the Chilean Comisión Nacional de Investigación Científica y Tecnológica (CONICYT). The observatory is managed by the Association of Universities for Research in Astronomy, Inc. (AURA) under a cooperative agreement with the NSF. The NSF also serves as the executive agency for the international partnership.